I recently moved into an old brick house (slate roof), containing several bathrooms without vents. It's a problem that I've been wanting to correct, though I've put it off because I'm not sure if knocking a hole in either the brick wall or slate roof is worth the hassle (esp. with respect to what I want from the system).

Since the fan's purpose is to move air out to remove humidity and odors, are there any fans out there that push the air through some type of replaceable desiccant to remove moisture, and charcoal to de-odor, so the air could be released back into the attic instead of the outdoors? To me, this seems like a reasonable option, considering I wouldn't have to worry about making holes in my bricks/roof (not to mention it eliminates the possibility of cold outdoor air flowing back in, which I hear is a problem even with dampered vents). Has anyone seen or used any such exhaust fans (do they exist)?

With respect to outdoor venting, the only option I can see, since it's a walk-up attic with small windows on the gable ends, might be to cut a circular opening in the glass that just fit the vent pipe (with damper from the other questions I've read) and finding a way to seal it properly might be an option (which means never opening the window). Is this an acceptable way to go about this?

Any suggestions and best practices for this old house would be appreciated!

  • How old is your old brick home? How high up is the brick wall portion that a proposed hole will be drilled for the vent location?
    – Jack
    Dec 26, 2013 at 4:03

6 Answers 6


Cutting a hole through brick is no big deal. You could use one of these as a disposable hole saw for $20: enter image description here

Or one of these as the "right" tool, which will be reusable for $75:
enter image description here

Most hole saws require a 1/2 inch drill plus a ~$10 arbor, both of which will be very useful for other projects in the future, like installing door knob sets in new doors, installing plumbing, ductwork, plenums, and all kinds of projects in plywood, sheet metal, lumber, plastics, etc.

When making the hole, take care to slant it slightly downward (2%—1/4 inch per foot—is typical) toward the outside to prevent rain, etc. from finding a path indoors. Install a vent device with a damper to prevent hot/cold air from coming inside when the fan is not on, and seal the hole at the outside surface thoroughly with silicon caulk.

Slate can be cut by this type of blade too, but holes through the roof are often fussier to make leak free.

  • Thanks! Seeing the necessary tools is very helpful and confidence building. Can you tell me what the $10 arbor is that you're referring to? Is that something that helps keep the drill steady and centered? Dec 26, 2013 at 18:45
  • Nix the arbor question-a quick search confirmed it's what I thought-I'd seen these but never knew what they were called. Thanks again! Dec 26, 2013 at 18:51

I don't see how a desiccant could possibly remove moisture rapidly enough to be helpful, and it also just traps the moisture instead of getting rid of it. Once the desiccant is saturated it won't absorb any more moisture (or it will just release it back into the attic — the whole problem you're trying to avoid). A dehumidifier with a drain in the attic near the exhaust outlet would be a somewhat better alternative, but even that is not going to keep the moisture from condensing in the winter.

If you have a window opening already, that may be your best bet. I assume you will have to remove part or all of the window to build an exhaust — just cutting a hole in the glass sounds fragile and leak-prone.

  • I concur on the desiccant. We tried it with a north facing bedroom in an attempt to deal with a mold problem and you end up with expensive bowls full of water and a minimal humidity reduction. Reinsulating walls, ceiling and floor, window replacement and most important now that heat loss wasn't a concern, changing the airflow for proper ventilation were what fixed the moisture problem. If you don't remove the moisture laden air, the moisture problem doesn't go away. No amount of desiccant does what proper ventilation does. Dec 25, 2013 at 22:55
  • Thanks for both quick responses. I'll have to get some numbers on air flow and adsorption rate to get a better gauge on this. I found some dessicant dehumidifiers though it would look like a kluge in the only place that might be close enough to be effective, between the sink and the shower/tub. A bedroom situation wouldn't necessary apply since there'd be no standing water in continuous drain mode available in the bathroom location. Still a tube running into the tub drain will look like a kluge. Dec 25, 2013 at 23:59

If you have access to the attic, another alternative is to vent through the eaves. You just have to make sure you get a damper designed for facing down or install the damper in a horizontal stretch of duct.

  • I have added this type of vent before, they are out there, and have damper on the outside face. 4 1/4" or maybe 3 1/4" hole saw for wood, ladder, is all you will need. Of course, drill, cord, among other things goes without sayin'.
    – Jack
    Dec 26, 2013 at 3:58
  • Thanks. I have attic access but I've found "rabid" squirrels here chewing on soffit wood and getting into the attic. I've had to put metal plates over the repaired wood, so I'm not inclined to make more holes in exterior wood, not knowing what triggers these squirrels acting like mega-woodpeckers. Dec 26, 2013 at 18:56
  • 1
    Sounds like you're rejecting perfectly good solutions. Vents can be made squirrel proof.
    – Bryce
    Dec 26, 2013 at 22:00

There is no quicker way to remove odors or moisture than a properly ducted exhaust fan. Surprisingly enough, it is not that difficult to break a hole through the brick and duct it out the side wall of the house, especially if you have a rotary hammer. Ducting through the attic is no issue either as long as if you are in a area that has cold winter climates, you will need to insulate the duct passing through any unconditioned space (the attic). You would also have to duct the exhaust fan right out of the attic through the roof. If the exhaust is piped directly into the attic you will have numerous moisture and mold issues especially if living in an area with cold climates.

There is a slightly more efficient way to remove humidity and odors. You can utilize a HRV or Heat Recovery Ventilator. What this does is remove humidity and odors from the air as well as expelling stale air from the house, and bringing in fresh air from outside thus ventilating the structure as well. Most HRV's also have a recirculation feature which will recirculate and remove humidity from the air without exchanging any air with outside of the structure. If you will be looking into one of these units i will recommend Venmar. They are the manufacturer of a popular HRV make here in Canada (Vanee) and have great build quality, along with functionality and serviceability.


Slate's actually quite easy to work with, don't take that off the table. Cutting a hole in brick is no big deal. Under eave vents could work, depending.

Chemical desiccant, however, won't work, as the moisture will never leave the building. You closest option is to distribute the moisture more evenly through the entire house through the HVAC system, but even that could readily raise the interior humidity too high.

A mechanical dehumidifier is a possibility, but only with a drain line.

  • Thanks, but the slate roof has already cost me many $K's in repairs since I moved in and even using only firms with good ratings on the consumer sites, I've had some workers who've caused more damage than repair not recognizing they have to be far more careful than the common asphalt shingles. Dec 26, 2013 at 18:27
  • See "slate roof central" to get hooked up with actual slate roofers.
    – Bryce
    Dec 26, 2013 at 21:59

If you have a 'cold roof' you can vent a 4" fan into the roof space, provided its large enough. My former flat ran with the arrangement in place for many years.

If there is a window have you considered a simple plastic ventilation fan or one of the motorised window varieties?

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